There are seldom substitutes for displacement. Torque is what gets you there faster.
What is your budget? How much per HP gained are you willing to spend?
Are you after track performance? Bragging rights? Noise?
Are you willing to swap engines? To a non-Porsche engine?
Willing to spend for the brake and suspension improvements to make 400HP safe? You can get there, it will just cost double or triple what you paid for your car.
The options you have are listed
here.
Lastly, what is the point of more HP in a car which will, in stock form, well exceed any legal limits. The Porsche line is about balanced performance, not drag strip numbers. I owned a 201HP Boxster 2.5 and a 254HP Boxster 3.2. The first was more fun. The second a better car but it was things other than the engine that made it better. Cooling, brakes, suspension, tires, transmission, etc.